1. Field of the Invention
The present invention relates to a drying apparatus and method and, more particularly, to a large scale industrial system and method for dehydrating fruits, nuts and other produce.
2. The Prior Art
Dehydration has long been used as a means of preserving certain types of fruits, nuts, and other produce. In addition, dehydration is also an important process in the preparation of certain types of widely used fruit products such as raisins, prunes, or the like.
Presently, there are a number of different ways which are used to effect large scale commercial dehydration of produce. For example, raisin growers have long used a method of dehydrating raisins which consists simply of spreading paper between the rows of grapevines and then picking the grapes and placing them on the papers on the ground between the rows. The grapes are then left over a period of several weeks and, during the heat of the day the grapes are dehydrated. The grapes must be periodically turned over by hand to insure that the product is thoroughly dehydrated and to minimize spoiling.
Obviously, this method has some very significant drawbacks. In the first place, the method is only usable in climates which are warm enough to accomplish the necessary dehydration. Second, the method suffers from the very serious disadvantage of being entirely dependent upon the weather. For example, if the dehydration process is interrupted by rain, most of the raisin crop may be lost. The method is further disadvantageous because it is not subject to control with respect to parameters such as temperature, humidity and the like. Thus, it is not possible to control with any degree of certainty the quality and uniformity of the raisins produced using this method.
Another system which has commonly been used by fruit and nut growers consists basically of a large wind tunnel in which the produce is stacked in trays up to a height of about six or seven feet. A very large, industrial type gas burner is placed in one end of the wind tunnel. When ignited, the burner heats the air at the incoming end of the tunnel and a fan is then used to force the heated air through the stacked trays of produce. As the heated air leaves the stacked produce it exits through the open end of the tunnel.
This type of dehydration system eliminates the uncertainty of having to rely on weather conditions for purposes of dehydration, and for this reason is a much more desirable system. However, this type of system also suffers from some substantial drawbacks. For one thing, with the rising cost of energy (for example, the cost of natural gas has doubled in the last two years), a tremendous expense is associated with the operation of the large, industrial size burners which are used to heat the air before it is forced through the stacked trays of produce. Indeed, many fruit and nut growers are finding the energy costs of this type of system to be so prohibitive that they are simply unwilling to bear these costs and have gone back to the more primitive method of dehydration as discussed above.
This type of system suffers from the further disadvantage that the air flow through the stacked trays may be very unevenly distributed. For example, if one tray in the stack is loaded with more fruit, air flow across that tray will be impeded. Thus, typically the air temperature will vary from the top of the stacked trays to the bottom of the stack and the moisture content of the air will also vary through the length of the stacked trays of produce. The results is that even with this type of system it is extremely difficult to produce raisins, prunes and other types of dehydrated fruits or nuts which consistently have a high quality. This further increases the waste and thus further reduces the cost effectiveness of this type of dehydration system.
What is needed in the art is a large scale, industrial system and method for dehydrating fruits, nuts or other produce which is highly energy efficient and which can also be controlled with greater precision so that a consistently high quality dehydrated product can be produced without undue expenditure of energy.